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Help !!! Delivery has failed to these recipients or groups
Delivery has failed to these recipients or groups: mailto:wichit@greenlatex.com Your message couldn't be delivered because you weren't recognized as a valid sender. The most common reason for th...
As Christopher mentioned it sounds like your account has been blocked by Microsoft due to suspected outbound spam. If you are an administrator for your Office 365 tenant you can remove the block (once the account has been secured). If not, you will need to get one of your admins to do it for you. The GUI method has changed a few times so the link may not get you where you need to be so if not, PowerShell is your friend. https://itspartlycloudy.com/2018/03/09/removing-multiple-blocked-senders-from-eop/